Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Enhanced electronic density

Liu, Z.-J., et al. (2003). Atomic resolution structure of obelin soaking with calcium enhances electron density of the second oxygen atom substituted at the C2-position of coelenterazine. Biochem. Biophys. Res. Commun. 311 433-439. [Pg.416]

The carhon-carbon double bond in alkenes is more reactive than carbon-carbon single bonds and gives alkenes their characteristic properties. As we saw in Section 3.4, a double bond consists of a a-bond and a 7r-bond. Each carbon atom in a double bond is sp2 hybridized and uses the three hybrid orbitals to form three cr-bonds. The unhvbridized p-orbitals on each carbon atom overlap each other and form a Tr-bond. As we saw in Section 3.7, the carbon-carbon 7r-bond is relatively weak because the overlap responsible for the formation of the 7r-bond is less extensive than that responsible for the formation of the a-bond and the enhanced electron density does not lie directly between the two nuclei. A consequence of this weakness is the reaction most characteristic of alkenes, the replacement of the 77-bond by two new a-bonds, which is discussed in Section 18.6. [Pg.858]

Wave functions can be calculated rather reliably with quantum-chemical approximations. The sum of the squares of all wave functions of the occupied orbitals at a site x, y. z is the electron density p(x,y,z) =Hwf. It can also be determined experimentally by X-ray diffraction (with high expenditure). The electron density is not very appropriate to visualize chemical bonds. It shows an accumulation of electrons close to the atomic nuclei. The enhanced electron density in the region of chemical bonds can be displayed after the contribution of the inner atomic electrons has been subtracted. But even then it remains difficult to discern and to distinguish the electron pairs. [Pg.89]

Electrophilic substitution reactions, typical of aromatic compounds with enhanced electron density, occur under relatively mild conditions, and exclusively at C-l and C-4. These reactions are summarized in earlier reviews... [Pg.836]

If the double bonds of a polyene are not conjugated with other 7r-systems in the molecule, addition to one of the tt-bonds will proceed in a similar fashion to addition to a simple alkene. Usually addition to one of the bonds is preferred, because it is either more highly substituted and, as a result, has enhanced electron density, or because it has fewer substituents and is less hindered and more accessible to the electrophile. Alternatively, one of the double bonds of a polyene may be activated by the presence of a heteroatom at the allylic position. [Pg.694]

Certain acetyl and benzoyl derivatives are surprisingly hydrolyzed in acidic media to the corresponding acid and the unsubstituted aza-pentalene (Table IV). This phenomenon appears to occur in compounds with an acyl group on a carbon atom where there is enhanced electron density. 280... [Pg.286]

The positron lifetimes for different defects in MgO are calculated using the insulator model of Puska and co-workers. In this model, the annihilation rates are determined by the positron density overlapping with the enhanced electron density that is proportional to the atomic polarizability of MgO [8, 9]. Based on comparison between experimental and calculated values [5, 6], the positron lifetime of the embedded Au nanoparticle layer, 0.41 ns, suggests that positrons are predominantly trapped in clusters consisting of... [Pg.331]

A variety of iodine compounds may be used as promoters, including l2. HI, Coil. CH >I, C2H5I, Phi, R4NI. R4PI. Nal. Kl, Csl. and Calj. [24. 29.30. It is interesting to note that the ionic iodides, with the exception of HI, arc inactive when used in combination with phosphines. Possibly, this is due to the enhanced electron density of phosphine-substituted cobalt carbonyls, which would make the nucleopliilic attack of iodide more difficult [24. ... [Pg.112]

The corresponding W complexes, (L—L)W(CO)4, are less reactive, and negligible reaction occurs in boiling THF. However, further exchange of a carbonyl group for a pyridine (i) introduces a ligand easy to displace and (ii) enhances electron density on the metal. Thus from complexes (L—L)M(CO)3py (M = Mo, W L—L = bipy, phen) (/ -allyl)M(CO)2(L—L)X is obtained where M is Mo as well as W. The coordinated-halide counter ion in these -allyl complexes can be replaced by ligands such as pyridine... [Pg.171]

Second, being quasibound Inside a potential barrier on the perimeter of the molecule, such resonances are localized, have enhanced electron density In the molecular core, and are uncoupled from the external environment of the molecule. This localization often produces Intense, easily studied spectral features, while suppressing non-resonant and/or Rydberg structure and, as discussed more fully below, has a marked Influence on vibrational motion. In addition, localization causes much of the conceptual framework developed for shape resonances In free molecules to apply equally well to photolonlzatlon and electron scattering and to other states of matter such as adsorbed molecules, molecular crystals, and Ionic solids. [Pg.140]

The [2.2]paracyclophanediene 42 with an enhanced electron density outside the aromatic rings has been used as a bidentate ligand for tricarbonylarenechro-mium complexes. The mono- and bis(tricarbonyl)chromium complexes 123 and 124 were obtained from the reaction of tris(propanenitrile)tricarbonylchro-mium with 42 and characterized by X-ray crystal structure analyses [83]. [Pg.117]

Enrichment on metals and metal-loaded sorbents is based on the formation of coordinative bindings between sorbents and analytes. Metals and metal-loaded sorbents have been described as suitable for the selective extraction of organic sulfur compounds because of the enhanced electron density at the analytes binding sites. [Pg.352]


See other pages where Enhanced electronic density is mentioned: [Pg.881]    [Pg.9]    [Pg.76]    [Pg.369]    [Pg.283]    [Pg.159]    [Pg.313]    [Pg.514]    [Pg.24]    [Pg.163]    [Pg.74]    [Pg.53]    [Pg.390]    [Pg.89]    [Pg.40]    [Pg.165]    [Pg.266]    [Pg.183]    [Pg.29]    [Pg.49]    [Pg.233]    [Pg.159]    [Pg.95]    [Pg.1229]    [Pg.164]    [Pg.764]    [Pg.462]    [Pg.390]    [Pg.181]    [Pg.343]    [Pg.109]    [Pg.22]    [Pg.38]    [Pg.91]    [Pg.139]    [Pg.452]   
See also in sourсe #XX -- [ Pg.3 ]




SEARCH



Density enhancement

Electronic enhancement

© 2024 chempedia.info